Superintendent Paul Langley talks to reporters at Horsley station near Guildford, Surrey, after a murder inquiry was launched following the stabbing of a man on board a train to London Waterloo (Image: PA)

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"What we know at the moment is that the victim boarded the train at around 1.00pm at London Road station in Guildford, along with his 14-year-old son.

"Traumatically, the victim's son would have been nearby when his father was fatally wounded.

"This would have been an horrific and hugely traumatic event to have witnessed and we are providing him with as much support as possible.

"We know that the suspect also boarded the service at London Road station, however at this time we do not believe they are known to each other.

"We also know that the suspect left the train service at Clandon station, and headed in the direction of Dedswell Drive. There have been a number of sightings in this areas from members of the public and our enquiries are focussing in on these areas.

"We have a number of officers in the area working hard to trace the man responsible for this violent and deadly attack. We are working close with Surrey Police and the National Police Air Service who are helping us to locate him.

"The suspect is a black man, in his twenties to thirties, approximately 6ft tall and of slim build with a beard, believed to be dressed all in black with white trainers.

"At this moment I would ask the public to please contact police urgently by calling 999 if they see anyone matching this description – please do not approach him.

"Our major investigation continues and we are appealing for witnesses to get in touch. If you were on board the train, we need to hear from you as soon as possible regardless if you saw what happened or not. Please text us on 61016.

"Finally, I would like to reassure the public that violent events such as this on the rail network are extremely rare.

"Over the course of the evening and into tomorrow, we will have extra officers out on duty in the local area to help reassure any concerned passengers and rail staff.